Production of powder metallurgical parts by preform and forge process utilizing sucrose as a binder

ABSTRACT

Metal particles are intimately mixed with a sufficient amount of sucrose to effect the desired degree of deoxidation and/or carburization. The mixture is poured into a mold and is then processed by (a) baking at low temperature to form a green compact with sufficient handling strength for further sintering and/or hot working or (b) heating to above sintering temperature to form a stronger compact similarly useful for hot forging.

United States Patent [191 Chao et al.

[ May 21, 1974 1 PRODUCTION OF POWDER METALLURGICAL PARTS BY PREFORM ANDFORGE PROCESS UTILIZING SUCROSE AS A BINDER [75] Inventors: Hung-ChiChao, Monroeville;

Robert R. Judd, Murrysville; Roger L. Rueckl, Murrysville; Charles K.Russell, Murrysville, all of Pa.

[73] Assignee: United States Steel Corporation, Pittsburgh, Pa.

221 Filed: Dec. 6, 1972 211 App1.No.:312,461

[52] U.S. Cl ..75/21l, 75/200, 75/20l,75/203, 75/204, 75/226, 264/111,156/336 [51 1 Int. Cl. B22f 1/00, B22f 3/00 [58] Field of Search264/111; 75/200, 201, 203, 75/204, 211, 226; 156/336 [56] ReferencesCited UNITED STATES PATENTS 2.158.845 5/1939 Ayer 156/336 2,176,30210/1939 Romp 75/204 2,509,838 5/1950 Oswald 2,279,003 4/1942 Matush3,006,859 10/1961 Allemann et a1. 252/30l.l

FOREIGN PATENTS OR APPLICATIONS 951,681 3/1964 Great Britain 264/63Primary ExaminerLe1and A. Sebastian Assistant Examiner-B. H. HuntAttorney, Agent, or FirmArthur J. Greif [5 7] ABSTRACT Metal particlesare intimately mixed with a sufficient amount of sucrose to effect thedesired degree of deoxidation and/or carburization. The mixture ispoured into a mold and is then processed by (a) baking at lowtemperature to form a green compact with sufficient handling strengthfor further sintering and/or hot working or (b) heating to abovesintering temperature to form a stronger compact similarly useful forhot forging.

7 Claims, 2 Drawing Figures L OOSE- PACK PROCESS AS- ATOM/ZED POWDERINT/MATELY MIX WITH .SUCROSE POUR MIXTURE IN TO MOL D FORM PREFORM BYSINTER/NG IN PROTECT] VE ATMOSPHERE FORM PREFORM DY BAKING INA/RPATENTEUHAY 2 1 197-1 38 1 1.878

saw 1 0r 2 CONVENTIONAL PROCESS AS- A TOM/ZED POWDER ANNEAL IN REDUCINGATMOSPHERE WITH CONCURRENT PRODUCTION OF CAKE GRIND CAKE TO ACHIEVEMETAL POWDER M/X WITH DIE LUBRICANT FORM PREFORM UNDER HGIH PRESSURE.SINTER IN PROTECTIVE ATMOSPHERE FORGE .SINTERED COOL T0 PRE'FORM //vROOM TEMP. CLOSED 0/5 REHEAT FOR FORGING' FORGE l/V CLOSED 0/5 FIG. I.

ATENTEDMAY 2 1 m4 SHEET 2 UF 2 LOOSE- PACK PROCESS AS- ATOM/ZED POWDER//V T/MATELY M/X WITH SUOROSE POUR MIXTURE INTO MOLD FORM PREFORM BYS/NTERl/VO IN PROTECTIVE ATMOSPHERE FORM PREFORM BY BAKING INA/R AT350-500F.

FORGE .Sl/VTEREO PREFORM IN CLOSED O/E COOL r0 HEAT FOR HEAT r0 .SINTERROOM TEMP. FORE/N6 a 70 ROOM 7544p.

05/1541 FOR 50005 nv 55/1547 FOR FORG/NG 040550 p FORG/NG 50005 IN FORGEnv. CLOSED 0/5 01.0550 0/5 PRODUCTION OF POWDER METALLURGICAL PARTS BYPREFORM AND FORGE PROCESS UTILIZING SUCROSE AS A BINDER This inventionis related to the production of powder metal preforms and isparticularly related to a process in which such preforms may be madefrom economical, as-atomized metal powders.

There are a number of different methods by which metal powders(particulate metals) useful in the production of powder metal compacts,have been produced. These methods include, for example, electrolyticprocesses, ore reduction processes and gas and water atomizationprocesses. The latter process has recently come to the forefront,especially in the production of ferrous metal powders, since the processis generally more economical and produces particles of a shape anddensity which provice a powder compact with enhanced physicalproperties. US. Pat. No. 3,325,277 is illustrative of a wateratomization process which is being commercially employed. In order toproduce a powder useful for further compacting, the asatomized powdermust first be annealed in a reducing atmosphere to soften the powdersand reduce the oxide surface thereof. As a result of this annealingprocedure, the particles tend to agglomerate and form a cake-likestructure, thereby necessitating an additional grinding stage tobreak-up the cake and finally achieve the desired particle shape andsize distributions required for further compacting. In the conventionalprocesses, these powders are then compacted under pressure and thenheated to elevated temperature to form the desired powder metal part or,in a more recent development, are similarly compacted under pressure andthen heated to elevated temperature to produce a preform, which is thenemployed for production of the final part.

It is therefore an object of this invention to provide a process bywhich high quality powder metal preforms can be produced from economicalparticulate metals such as as-atomized metal powders.

Another object of this invention is to eliminate the limitations imposedby practically sized compacting presses, in the production of powdermetal preforms.

Still another object of this invention is to provide a process whichenables the use of relatively inexpensive and expendable molds in theproduction of powder metal preforms.

These and other objects and advantages of the invention will be moreapparent from the following description and appended claims when takenin conjunction with:

FlG. I which is a flow diagram of the conventional process for theproduction of powder metal preforms, and

FIG. 2 which is a flow diagram of the basic embodiments of thisinvention for the production of powder metal preforms.

It has now been found that as-atomized powder can be directly employed,if the powder is initially admixed with sucrose, which serves to (a)reduce the oxidized surface of the powder, (b) act as a carburizingagent to achieve the desired carbon content in the powder metal preform,and in a further embodiment, (c) act as a binder when heated to lowtemperatures, serving to provide a green preform which may be handledand transported for further processing. It may be seen in comparingFIGS. 1 and 2, that utilization of sucrose in combination with theoutlined procedures permits the elimination of both the annealing andgrinding steps of the conventional process. Additionally, a number offurther benefits are achieved by following the procedures of thisinvention. Referring to FIG. 1, it may be seen that in the conventionalprocess, the preform is produced by admixing the annealed and groundpowders with a lubricant, and then compacting under high pressures,generally in excess of 30 tons per square inch. Utilizing such aprocedure, it is necessary that fully processed (annealed and ground)powder exhibiting a considerable degree of irregularity of particleshape be employed to insure adequate strength for handling afterpressing. The resulting green preform is then sintered under aprotecting atmosphere at temperatures of about 2,000 F. In somecommercial procedures the pressing and sintering are accomplishedsimultaneously. This procedure has not received significant commercialutilization, because of the severe limitations imposed by the necessityof providing die materials which exhibit very high strength at ratherelevated.

temperatures.

In contrast with these conventional procedures (i.e., embodiment (I) ofthe instant invention) the blended mixture of powder metal and sucroseis poured into a ceramic or metal mold, preferably vibrated to a bulkdensity substantially in excess of apparent density, and then heated at1,2002,400 F in a protective atmosphere to effect annealing andsintering in one step. For purposes of this invention, the termsintering is directed to the joining together of metal particles/by theapplication of heat in the absence of substantial ex- I ternalpressures, i.e. pressures in excess of l tsi. In view of thissinteringof the as-atomized powder in combination with sucrose, thecarbon reducible oxides (e.g. various forms of iron oxide as well as theoxides of nickel, copper, molybdenum, etc.) of the powder are reducedand the metal powders softened in a manner analagous to that achieved inthe annealing step of the conventional process. Since this sinteringproduces a preform with good green strength, the grinding and pressurecompaction procedures of the conventional process are clearlyunnecessary. Thus, additional economies are realized through theelimination of the rather expensive high-pressure press. Of equalimportance, the attendant size limitations of the preforms made byconventional process are eliminated. In the conventional process, thepressed preforms are limited (at least in a practical sense), by thesize of available presses, to the production of relatively smallpreforms, generally less than 10 pounds. In contrast, significantlylarger preforms, ranging up to several hundred pounds, may be sinteredby the instant process and then forged to the desired part. Finally, thelower density of the sintered only preform, permits better metal flowduring forging, resulting in both significant reductions of the energyrequired for forging and in better die filling characteristics.

In the second embodiment (II) of this invention, the' as-atomized metalpowder-sucrose mixture is poured into a mold and baked at a temperature(generally 350500 F) sufficient to soften the sucrose and thereby form acohesive green preform. The relatively low-temperatures which may beemployed in this baking procedure, allows the use of a variety ofinexpensive, expendable mold materials such as various plastics orrubbers or even paper; the only requirement being that the mold materialbe capable of withstanding the rather low baking temperature. Therefore,while ceramic or metal molds may be utilized. the full economic benefitsof this embodiment will be realized by utilizing such inexpensive,expendable molds. Ceramic molds present a further problem in that it isoften difficult to remove the preform without the necessity of specialprecautions being taken. After the cohesive baked preform is dischargedfrom the mold, it may be processed by either of two alternative routes,dependent primarily on equipment availability and the size of thepreform. In the first of these routes, the preform is heated in aprotective atmosphere and forged in a manner similar to the conventionalpreform and forge process. In the second route, the baked preform issintered (heating for at least minutes at temperature, preferablyl,800-2,200 F) in a protective atmosphere and then forged directly,making use of the sensible heat of sintering; or cooled and thenreheated for forging at a later time.

In experiments leading to the instant invention, a variety of potentialcarburizing materials were evaluated for their effectiveness inproviding a suitable binder. in the tests reported below, all thebonding agents were essentially of the same particle size, i.e., minus200 mesh. The metal powder was all minus 6 mesh and had the followingscreen analysis:

Mesh Size Percent Retained 80 I8. l I00 2.0 I40 4.3 200 I 3.2 230 4.0325 [06 pan 47.8

The metal powder-binder combinations were blended and poured into thepreform mold, which was mechanically vibrated to achieve a bulk densitysubstantially in excess of apparent density.

4 strength to be easily removed frornthe fiioldand handled.

Further tests were conducted to determine if more uniform coating of themetal particles could be achieved by use of solutions of sucrose inwater. Surprisingly, no improvement in distribution was achieved. Moreimportantly, it was determined that any substantial percentage ofmoisture was, in fact, detrimental. Thus, at low levels of about 1 to 5percent moisture, the metal powder-sucrose mixture would not flowproperly even when vibrated, thereby resulting in an incompletely filledmold. At higher water levels, the mixture did effectively fill the mold.However, this necessitated an extra step of preliminary drying with theattendant requirement for the taking of rather impractical precautions.Thus, drying had to be achieved at a temperature below 212 P, so thatthe packed powder was not disturbed by the water boiling-off. Dryingtherefore became a lengthy and time consuming process, primarily becauseof the small exposed surface area of the powders in the mold. Even withsuch preliminary drying, it was found that the baked preforms did notachieve the same high density as those made with essentially drymixtures. It is therefore preferable that the metal powder-sucrosemixture be essentially dry, i.e. less than 0.5

percent moisture.

In general, the features of the instant invention are applicable tometal powders or particles from virtually any source. However, a fewinstances do exist in which one or the other of the two embodiments maybe ruled out as inapplicable to the desired objective. For purposes ofunderstanding the applicability of these embodiments, source powders maybe divided into two categories: (a) relatively pure metal powders withcarbon reducible oxygen contents below about 200 ppm (e.g., inert gasatomized powder, electrolytic powders, rotating electrode powders) and(b) metal powders or particles with carbon reducible oxygen contents substantially in excess of 200 ppm (e.g., as-atomized pow- TABLE 1 WeightBaking conditions of binder Binder type (percent) Temp. (F) Time (min.)Results Dextrose 2.5 400 Stuck to mold. no strength, could not behandled.

5.0 400 60 Do. 2.5 550 60 Do. 5.0 550 60 Do. 5.0 400 90 Do. 5.0 550 90Do. Lactose 2.5 400 60 No bond. remained powder.

5.0 400 60 Slight bond, however, could not be handled. 2.5 400 90 Nobond, remained powder. 5.0 400 90 Slight bond, however, could not behandled. 2.5 550 90 No bond. remained powder. 5.0 550 90 Slight bond.however. could not be handled. Maltose 2.5 400 60 No bond.

5.0 400 so Developed some bond, but softened after cooling. somesticking to mold. 2.5 400 90 Very slight bond. could not be handled. 5.0400 90 Developed some bond, however. softened on cooling.

stuck badly to the mold. 5.0 550 90 Binder ran to bottom of mold. verysevere sticking to mold. Potato starch 5.0 400 90 No bond 5.0 550 90 Do.Methyl cellulose..... 5.0 550 90 Do. 5.0 550 90 D0. Sucrose 2.5 400 60Excellent bond. no sticking. adequate strength for all handling. 5.0 40060 Do.

It may be seen from the above, that irrespective of der, mill scale).For purposes of this invention, carbon binder concentration and bakingtemperature, only sucrose provided a baked, green preform which did notstick to the mold and which exhibited sufficient reducible oxygen ismeant to include those metal oxides which are capable of being reducedby carbon at temperatures below about 2,400 F. As stated hereinabove,the admixture of the metal powder with sucrose serves to reduce theoxidized surface of the powder, act as a carburizing agent and inembodiment (ll), act as a binder when baked at low temperatures. Thus,if pure metal powders of category (a) are employed, and there is norequirement for the carburization thereof, only the baked preform route,i.e., embodiment (II) would be applicable. In this instance, the carbonwould then be removed as a result of heating in a controlled atmosphereduring sintering and/or prior to forging. Similarly, there are instanceswhen it is only desirable to increase the carbon content by as little as0.04 percent. If relatively pure powders are employed (no attendantoxygen reduction), the amount of sucrose added in such a case, will beinsufficient to act as an effective binder in the production of a bakedpreform, i.e. route II, and only the sinter preform embodiment would beapplicable. However, for the production of most powder metallurgicalparts, it is generally desirable to effect significantly greaterincreases in the carbon content of the starting powders (e.g. 0.2percent). Thus, in many cases, even when pure iron powders are employed,the required amount of sucrose will be sufficient to permit theutilization of both embodiments of this invention.

Although applicable to pure metal particles, the instant procedures areof particularly notable advantage when employing metal particles ofcategory (b), i.e. those with carbon reducible oxygen contentssubstantially in excess of 200 ppm. If the latter type particles crose,when employed in a relatively pure state, preferably less than 2 peicentash content, exhibits an exceedingly high and uniform reactivity,approaching that of the better natural graphites.

The ferrous metal powder-sucrose combination is intimately mixed, i.e.,by blending, to achieve a uniform distribution; poured into the mold;vibrated to increase density and then baked at temperatures in excess ofabout 350 F, to glue the particles together and achieve sufficient greenstrength for further processing. At least about 1.5 wt. percent sucroseis required to achieve a baked preform with sufficient handlingstrength. Typically, 'water atomized ferrous powders (with carbonreducible oxygen contents of 1,000 to 20,000 ppm) require the addition.of from about 2 to 10 percent sucrose. For economic reasons, the bakingis generally accomplished in air; in which case temperatures in excessof about 500 F are undesirable due to excessive carbon oxidation.Obviously, no such temperature limitation is imposed, if the baking isaccomplished in a non-oxidizing atmosphere.

The method above was employed for the production of a differential gearand test bars, from a modified TABLE II.COMPOSITIO N OF MODIFIED 4600GRADE STEEL EVALUATED-PERCENT BY WEIGHT C Mn P S Si Cu Ni Cr Mo Al NTotal 0 are employed it is desirable to know the oxide content (i.e.hydrogen loss) of the particles, since it is first necessary that thesucrose reduce the oxides before it can effectively combine with theiron powder. Thus, the amount of sucrose which is added is dependent onboth the hydrogen loss of the particles and the desired carbon contentof the final part. With a knowledge of the hydrogen loss of theparticles, it would of course be possible to calculate thestoichiometric amount of sucrose required to achieve such a desiredfinal carbon content. However, it is preferable that the required amountbe determined empirically, since it has been found that the efficiencyof carburization is, to a large extent, affected by the characteristics(e.g. grain size, shape) of the powders employed.

In the recarburizing of iron powders, it is already known in the artthat even when sufficient amounts of carburizing agent are employed,that the mechanical properties of the final product are stronglydependent on the reactivity of the carburizing agent. Thus, lampblacks,carbon blacks and synthetic graphites exhibit poor reactivities and aregenerally considered unsuitable as carburizing agents for the productionof powder metal parts with optimum mechanical properties. Even thenatural graphites vary considerably in the reactivity they exhibit.Surprisingly, it hasbeen found that suempirically determined that 3.2wt. percent sucrose was required for this particular powder. The blendof powder metal and sucrose were poured into a mold, vibrated toincrease density and baked in air at 400 F for about 40 minutes. Aftercooling, the baked preform was removed from the mold and sintered in ahydrogen atmosphere at 2,050 F for 30 minutes. The baked and sinteredpreform was cooled and shipped to another facility for furtherprocessing, which comprised heating the preform inductively (in anatmosphere of 5 percent H percent N to various temperatures within therange of 1,200 to l,700 F. The heated preforrns were then immediatelyforged at about 60 tons/in and then air cooled. The resultant mechanicalproperties of the so forged test bars are shown in Table III.Noteworthy, is the relatively high ductility and good notch toughnessachieved, especially in view of the significant costreductions realizedusing the instant process. The differential gears were then furtherevaluated in the drift-pin test. In this test, a tapered, hardened steelpin is pressed into the bore of the gear until failure occurs. If thegear sustains a load of 20,000 pounds without failure, it is consideredsatisfactory. Shown in Table IV are the results obtained under a varietyof forging conditions. Even the gears forged at comparatively lowtemperature and pressure, passed the test.

TABLE lII.-ROOM-TEMPERATURE ME(HANICAL PROPERTIES OF PREFORMED ANDFORGED TEST BARS MADE FROM MODIFIED 4600 GRADE STEEL Yield strengthFracture 10.2% Tensile Elongation Reduction Average Energy Lateralappearance offset) strength i l i nch of area hardness absorber?expansiom (percent C di i (ks tksi) (Percent) (percent) n) (ft-lb)(mils) shear) Test bars forged from 6-mesh powder As-forged' 86.1 94.221 4 89 30 39 I Heat-treated 90.4 I 09.0 I 2 26 97 32 35 I00 Test barsforged from -80-mesh powder As-forged' 79.3 91.9 27 52 92 30 32 I00Heat-treated 89.4 I 09.0 I8 57 95 38 43 100 Cha V-notch test resultswith standard size specimens. A 'PX A.

' Test bars were stress-relieved for one hour at I000F before testing. 2Test bars were austenitized for one hour at I600F, oil-quenched and thentempered for one hour at 800F.

TABLE IV.RESULTS OF DRIFFPIN TEST ON MODIFIED 4600 GRADE STEEL GEARSFORGED FROM PREFORMS MADE BY LOOSE-PACK PROCESS Increase Forging MaximumPin Energy to in bore Gear temperature load (I000 displacement failure(I000 diameter designation (F) pounds) linches) inch-pounds) (percent) aI660 36.5 1.65 3i.2 22 b.. I660 39.8 3.00 76.2 43 0... I660 40.9 2.8276.2 38 d... I545 45.3 2.65 72.6 36 e... 1575 2L0 l.80 23.8 24 f....I510 33.5 I94 30.7 25 g... I565 25.6 I66 25.2 22 h... I555 22.9 1.9028.5 25 I650 24.] 2.60 43.9 34 j I343 26.5 1.36 21.8 18

Note: All gears were forged with a 4 to I die-lubricant-water mixtureexcept for gear j, for which an 8 to I mixture was used. The gears werestress-relieved for one hour at I000F before testing.

heating the filled mold to a temperature of at least about 350 F, butsubstantially below that at which said metal particles will sinter, saidheating being conducted for a time at least sufficient to soften saidsucrose to form a baked preform with sufficient strength for handlingand further processing.

2. The method of claim 1, wherein said metal particles are ferrous basemetal powders with a carbon reducible oxygen content substantially inexcess of 200 ppm and said sucrose is present in an amount sufficient toreduce said oxygen and increase the carbon content by a value greaterthan 0.2 percent, during the carburization of said ferrous particles.

3. The method of claim 2, wherein said heating is accomplished in air ata temperature below about 500 F.

4. The method of claim 3, wherein said blended mixture is essentiallydry and contains from about 2.0 to 10.0 percent sucrose.

5. The method of claim 4, wherein the particles in said mold are packedto a bulk density substantially in excess of apparent density.

6. The method of claim 5, wherein said mold is composed of aninexpensive, expendable material with sufficient refractoriness towithstand said heating temperature.

7. The method of claim 6, wherein said baked preform is cooled andremoved from said expendable mold.

2. The method of claim 1, wherein said metal particles are ferrous basemetal powders with a carbon reducible oxygen content substantially inexcess of 200 ppm and said sucrose is present in an amount sufficient toreduce said oxygen and increase the carbon content by a value greaterthan 0.2 percent, during the carburization of said ferrous particles. 3.The method of claim 2, wherein said heatIng is accomplished in air at atemperature below about 500* F.
 4. The method of claim 3, wherein saidblended mixture is essentially dry and contains from about 2.0 to 10.0percent sucrose.
 5. The method of claim 4, wherein the particles in saidmold are packed to a bulk density substantially in excess of apparentdensity.
 6. The method of claim 5, wherein said mold is composed of aninexpensive, expendable material with sufficient refractoriness towithstand said heating temperature.
 7. The method of claim 6, whereinsaid baked preform is cooled and removed from said expendable mold.